1. Field of the Invention
The present invention relates to a MOS transistor and a MOS transistor manufacturing method, and more particularly, to a MOS transistor having a wider resistor linearity range and the manufacturing method thereof.
2. Description of the Prior Art
A voltage-controlled resistor is an element whose resistance can be changed with an input control signal and is widely applied in a variety of integrated circuits such as tuning circuits. In general, when the relationship between a current value of the current flowing through a resistor and a voltage drop between two ends of the resistor is linear, the resistor is regarded as a resistor of good characteristic. In other words, when the resistance stays constant in an operation region, the resistor is regarded as a resistor of good characteristic. When a MOS transistor, namely a metal oxide semiconductor field effect transistor (MOSFET), operates in a triode region, the relationship between the drain-to-source current Ids and the drain-to-source voltage Vds is linear, and it is usually used as a voltage-controlled resistor. However, when the MOS transistor is used as a voltage-controlled resistor, its operation range is restricted. Relevant descriptions are as follows.
Please refer to FIG. 1. FIG. 1 is a schematic diagram illustrating a structure of an N-type MOSFET produced using a standard process. The typical N-type MOSFET includes a substrate 100, a gate 120 located on the substrate 100, two N-type source/drain doped regions 104 located in the substrate 100 and set at two sides of the gate 120, and spacers 160 disposed over two sides of the gate 120. In addition, a lightly doped drain (LDD) 106 and a halo doped region 108 are arranged in the substrate 100 below two sides of the gate 120 respectively.
The drain-to-source current Ids of the N-type MOSFET can be represented by the following equation:
                    Ids        =                  μ          ⁢                                          ⁢          Cox          ⁢                                          ⁢                                    W              L                        ⁡                          [                                                                    (                                          Vgs                      -                      Vth                                        )                                    ⁢                                      Vds                    ′                                                  -                                                      1                    2                                    ⁢                                      Vds                    ′                                    ⁢                  2                                            ]                                                          (        1        )            
Although a small voltage drop between the two sides of the LDD 106 is induced through the existence of the LDD 106, Vds′≅Vds can be assumed since the voltage drop is negligible. When Vds<<2(Vgs−Vth) (i.e. a MOSFET operates in a triode region), the above equation (1) can be rewritten as shown below:
                    Ids        ≅                  μ          ⁢                                          ⁢          Cox          ⁢                                          ⁢                                    W              L                        ⁡                          [                                                (                                      Vgs                    -                    Vth                                    )                                ⁢                Vds                            ]                                                          (        2        )            
If the path from the source to the drain serves as a resistor, the relationship between the drain-to-source current Ids and drain-to-source voltage Vds is linear, and the resistance can be represented by the following equation:
                    Rout        =                  1                      μ            ⁢                                                  ⁢                          C              ox                        ⁢                          W              L                        ⁢                          (                                                V                  gs                                -                                  V                  th                                            )                                                          (        3        )            
In light of equation (3), it is observed that the typical MOSFET can be used as a voltage-controlled resistor because the resistance between the source and the drain can be controlled easily by adjusting the gate voltage.
However, in order to keep the resistance between the source and the drain of the typical MOSFET constant, the operation range of the drain-to-source voltage Vds is very narrow. Once the voltage value of the drain-to-source voltage Vds gets too high, the MOSFET will leave the triode region, and the resistance between the source and the drain will no longer remain constant. Therefore, the characteristic of the whole circuit will be affected. For example, when an input AC signal swings over the operation range of the drain-to-source voltage Vds, signal distortion or signal skew of the input signal will arise.